Weirs and Canal
Intakes
By: Nell Kolden and Taylor Barnett
Thursday April 12, 2012
CIVE 717
Weirs and Canal Intakes
• Objectives
• To divert water from main canal into secondary canals
• To help maintain a more uniform flow rate over long periods of
time
• To measure flow rates
• To maintain a constant minimum flow depth
• To control grade of river
Weirs and Canal Intakes
• Main Types of Weirs
• Sharp Crested Weir
• Broad Crested Weir
• V-Notch Weir
• Main Types of Canal Intakes
• Lateral Intake
• Bottom Intake
• Overhead Intake (Inlets in piers are used for water intake)
Weirs and Canal Intakes
• Lateral Intake with damming
• Uses a Weir or similar hydraulic control structure to maintain a
minimum stage
• Allows the off take structure to continue operating regardless of
river conditions
• Harder to design and build than a lateral intake without damming
• Will have a higher environmental impact than a lateral intake
without damming
Weirs and Canal Intakes
• Example of a lateral intake with damming
Weirs and Canal Intakes
• Lateral Intake without damming
• Off take flow rate is limited to 1 or 2 cms assuming 2cms<<Qriver
• Off take flow rate is directly tied to river flow rate since there is
no check structure to ensure a minimum river stage
• Avoids generating backwater and is cheaper to construct
Weirs and Canal Intakes
• Example of a lateral intake without damming
Weirs and Canal Intakes
• Bottom Intakes
• Also known as Tyrolean Intakes
• Water is diverted from river using a collection canal
perpendicular to the river and covered with a screen
• Particles and debris larger than the screen are washed
downstream
• Flow rate in off take canal will always be at maximum so this is
best suited for power production
Weirs and Canal Intakes
• Sharp Crested Weirs
• Considered to be a sharp crested weir when
, where y1 is
the upstream flow depth above the weir, and l is the weir length
• Can be used as check structure or to measure flow rates
• If used to measure flow rates:
• Where H1 is the total energy upstream of the weir
• The weir is considered submerged if
• If the weir is submerged the discharge
equation is multiplied by f(S)
Weirs and Canal Intakes
• Broad Crested Weirs
• Considered to be a broad crested weir when
, where H1
is the upstream total energy above the weir, and l is the weir
length
• Can be used as check structure or to measure flow rates
• If used to measure flow rates:
• Where H1 is the total energy upstream of the weir
• The weir is considered submerged if
• If the weir is submerged the discharge
equation is multiplied by f(S)
Weirs and Canal Intakes
• Sharp Crested Example Un-submerged
• Sharp Crested Example Submerged
Weirs and Canal Intakes
• Broad Crested Weir Example
Weirs and Canal Intakes
• V-notch weirs
• Found all over the world because construction and installment is
very straightforward
• Used primarily as a simple flow measurement device
• A relationship between flow height through the notch and
discharge is easily developed and measurable in the field
• Q=(g/2)1/2*yc5/2
• Where: Q is discharge, g is gravity, and yc is the height of the water
through the notch
V-notch weir used for flow measurement
Weirs and Canal Intakes
• Weirs can also be used to improve fishing conditions
• Fishing weirs found all over the world, some are very simple and
some involve complex engineering
• Backup of water upstream of weir creates a holding place for fish
Fishing weir used to trap fish in a bay near Taiwan
Weirs and Canal Intakes
• Bendway weirs are used to redirect flow in meander bends in
rivers
• The objective of the bendway weirs is to improve navigation
through tight bends.
• Usually angled 20°-30° upstream
• The weirs are low enough that ships can pass even in low flow
• They are high enough so that they significantly control nearbed velocities and redirect near-bed streamlines
• Built in sets of approximately 4-14 weirs per bend
Weirs and Canal Intakes
• Bendway Weir:
Weirs and Canal Intakes
• Recreational uses:
• Weirs are often built to create pools for swimming and recreation
Swimmers enjoying the pools created by a weir
• The water drop created by a weir on the downstream side can be
used for boating and tubing
Kayaker surfing downstream of a weir
Weirs and Canal Intakes
• Habitat considerations:
• A large drop off the downstream edge of a weir can create a
barrier to upstream fish passage
• Weirs can be built so that they maintain or enhance fish habitat
• Rock ramps on the downstream end of weirs create riffle habitat
instead of a small waterfall
This weir on the South Platte creates a
backwater condition to slow velocities and
prevent bridge pool scour, but also creates riffle
habitat and does not impede upstream fish
passage
Weirs and Canal Intakes
• One type of habitat-enhancing weir is a Newbury Riffle
• Newbury Riffles are installed as a combination grade control
structure/habitat improvement
• The weir backs up water into a pool, and the riffle maintains
fish passage
Series of Newbury Riffles
Weirs and Canal Intakes
• Weir Construction:
• One simple way to construct a weir is to use sheet piles driven
into the river bed to a depth 2-3 times the maximum scour rate
• Regardless of the construction material, the sides of the weir
must be tied into the banks with riprap or sheet pile to keep the
water from getting around the weir
• For weirs with ramps on the downstream side, rocks must be
placed carefully to maintain desired slope over backside of weir
• When multiple weirs are used for grade control, they must be
constructed so that the energy slope in between structures
matches that needed for sediment control
Weirs and Canal Intakes
• Weir Failures:
• When weirs are not adequately constructed or not tied back into
the bank, failure can occur
• Failed weirs can cause dangerous situations for animals,
navigation, and recreation
Failed concrete weir on Clear Creek
Weir lacking adequate tie-back
Weirs and Canal Intakes
• Conclusions:
• Weirs and Canal Intakes have a huge range of uses and designs
• They must be designed properly so that they function as intended
The triple weir on the Avon River in Bath (There
is a lock on the right side to allow for navigation)